This invention relates generally to methods utilizing sonic tools for investigating the properties of earth formations traversed by a borehole. More particularly, this invention relates to the investigation of the velocity profile and extent of a perturbed zone surrounding a borehole in an earth formation via the use of array sonic borehole tools.
Before a borehole is drilled, virgin formation rocks are under stress due to overburden pressure. The removal of rock in a formation by the drilling of a borehole has a radial effect on the remaining formation around the borehole in the form of stress relief. The distribution of the stress components (i.e. radial, and angular) in the medium around the borehole varies depending on the type of rock which comprises the medium and the distance of the rock from the borehole, as well as e.g., the borehole pressure, mud weight, rock strength, pore pressure, cohesive strength, and the Poisson's ratio of the rock. Determinations of rock strength, pore pressure, and rock stress are all useful in predicting over-pressure zones, sanding and fracturing, all of which are critical in managing the production of hydrocarbons from the formation.
For the situation where the borehole pressure is greater than the in situ stress, the radial stress is at a minimum and the angular stress is at a maximum just outside the borehole wall in hard formations which do not crumble. However, in less hard formations such as unconsolidated sandstones, when a borehole is drilled, the rock surrounding the borehole crumbles and stress in the rock is redistributed. As seen in the prior art FIG. 1, the principle stress components .sigma..sub.r, .sigma..sub.z, and .sigma..sub..THETA. in such a formation assume a profile where they are a function of radial distance rom the borehole wall. In the "plastic" zone which is closest to the borehole wall, the stress components .sigma..sub.z, and .sigma..sub..THETA. are essentially equal, while in the virgin zone which is unaffected by the borehole, and in the transition zone (which bridges the plastic zone and the virgin zone), each of the stress components assumes a different value.
In determining the stress profile of a disturbed zone (i.e., the plastic and transition zones), it is known that array acoustic tools may be used to determine a velocity profile and that the velocity profile will in turn yield a stress profile. Ray tracing techniques measuring transit times, and dispersion techniques measuring different phase velocities at different frequencies have been utilized; See, e.g. Brian Hornby and S. K. Chang, "A Cased Study of Shale and Sandstone Alteration Using a Digital Sonic Tool", Proceedings of SPWLA 26th Annual Logging Symposium; Dallas, Tex. (Jun. 17-20, 1985). While effective in some formations, these techniques are best suited to situations where velocity contrasts between acoustic waves at the borehole wall and in the virgin formation are large. The techniques, however, are not as effective in sandstones where velocity contrasts are small.
Besides using ray tracing techniques and dispersion techniques for measuring velocities, it is known that the amplitude of a compressional wave arrival varies in relationship to the thickness of the altered zone as well as in relationship to velocity contrast between the virgin and altered formations; see Tubman, Kenneth M.; "Effects of Borehole Alteration on Amplitudes of Full-Waveform Acoustic Logs" Borehole Geophysics II; Extended Abstract, SEG 55th Annual Meeting, Washington, D.C. (Oct. 6-10, 1985) pp. 46-46. The Tubman article suggests that there is a qualitative relationship between compressional wave arrival amplitudes and altered zone velocities, but does not provide or suggest a manner for making a quantitative determination.